Role of Potassium Exchange in Catalytic Pyrolysis of Biomass over

Research Article pubs.acs.org/journal/ascecg

Role of Potassium Exchange in Catalytic Pyrolysis of Biomass over ZSM-5: Formation of Alkyl Phenols and Furans Charles A. Mullen,* Paul C. Tarves, and Akwasi A. Boateng USDA-ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States

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S Supporting Information *

ABSTRACT: Catalytic fast pyrolysis of biomass with ZSM-5 type zeolites is a commonly considered in situ upgrading technique for the production of partially deoxygenated bio-oils. The acidity and structure of ZSM-5 catalysts favor the production of aromatic hydrocarbons from oxygenates present in the pyrolysis vapors, such as acids, anhydrosugars, ketones and aldehydes. However, these acid catalyzed deoxygenation pathways remove functional groups from the aromatic ring structure, which makes the liquid bio-oil more amenable to processing in current petroleum refineries while subsequently decreasing the value of the compounds as industrial chemical feedstocks. The observation of improved yields of alkyl phenols and furans during catalyst deactivation suggests a method for tuning the product distribution by altering the parent zeolite catalyst via changes in acidity and/or incorporation of alkali metals known to accumulate on the catalyst during pyrolysis. Here, we report the catalytic fast pyrolysis of three biomass components (cellulose, xylan, and lignin) and switchgrass with two different HZSM-5 catalysts and their corresponding potassium exchanged counterparts (KZSM-5). The catalyst:biomass ratio and pyrolysis temperature were optimized for the production of monomeric phenols. The KZSM-5 provided ∼3−4-fold increases in the yields of both alkyl phenols and 2-methylfuran when compared to the noncatalytic and high acidity HZSM-5 catalyzed experiments while decreasing the yield of monoaromatic hydrocarbons. The observations made in the course of this study suggest that the main role of Kexchange was to attenuate the Brønsted acidity of the zeolite and show that zeolites can be tuned to produce specific classes of compounds for use as renewable chemical feedstocks and can be used to develop catalytic conditions to further improve the yields of these valuable chemicals. KEYWORDS: HZSM-5, K-ZSM-5, Biomass, Pyrolysis, Alkyl-phenols, Furans

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decomposition of the catalyst.4,12 Although coke deposits can be removed from the catalyst via combustion, deactivation due to alkali metal deposition is not easily reversed and can be a particular problem for herbaceous biomass species that tend to have more inorganic components than woods.12 Many groups have looked to produce modified versions of the HZSM-5 catalyst to both increase the yield of aromatic hydrocarbons and decrease coke formation in order to increase catalyst lifetimes. Methods have included modifying acidity through Si/Al ratios,14 incorporating metals such as Ga,15−19 and changing the pore size distribution.20 Few studies have focused on the products formed as the catalyst deactivates. The product mixture formed using a partially deactivated catalyst is not simply a mixture of aromatic hydrocarbons and primary oxygenated pyrolysis products. It also contains some oxygenated compounds in higher concentrations than found in products from fresh catalysts and noncatalytic pyrolysis.13,21 In

INTRODUCTION Thermochemical conversion of biomass, particularly pyrolysis methods, has received a great deal of attention because it offers inexpensive routes to efficiently liquefy and densify lignocellulosic biomass, which is the most abundant source of renewable carbon.1 The liquid bio-oil that results from pyrolysis is a complex mixture of oxygenated organic compounds that contains many potentially useful and valuable chemicals.2 However, because of the low concentration of any one compound, the difficultly in separating and/or processing the thermally unstable mixture, and because of the focus on production of renewable drop-in fuels, much of the research has concentrated on complete defunctionalization of the bio-oil either through postproduction upgrading (i.e., hydrodeoxygeantion) or pyrolysis vapor upgrading techniques.3 Among the later methods, aromatic hydrocarbon production via catalytic pyrolysis over ZSM-5 type zeolites has been one of the most extensively studied processes.4−13 One of the major drawbacks of this process is rapid catalyst deactivation, which can happen through a variety of mechanisms including internal and external coke deposition, poisoning by alkali metals, or physical © 2017 American Chemical Society

Received: September 19, 2016 Revised: January 4, 2017 Published: January 22, 2017 2154

DOI: 10.1021/acssuschemeng.6b02262 ACS Sustainable Chem. Eng. 2017, 5, 2154−2162

ACS Sustainable Chemistry & Engineering

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particular, alkyl phenols and furans can be found in higher concentrations when the catalyst begins to deactivate.13 These observations may allow for an opportunity to design catalysts that produce both aromatic hydrocarbons and oxygenates that are potentially more valuable and more easily isolable than the products found in traditional fast pyrolysis oils. The value of producing functionalized oxygenates can be illustrated by a comparison of the value of benzene and its hydroxyfunctionalized analog, phenol, where in the current market phenol commands a >$300/mt premium over benzene.22 Alkyl phenols may be used in plastics, pharmaceuticals, and cosmetics. Alkyl phenols have also proven to be easily separable from stable bio-oil mixtures via extraction and/or distillation.23,24 Similarly, furans are more valuable than most defunctionalized hydrocarbons. Tetrahydrofuran (hydrogenation product of furan), a common organic solvent, sells for more than three times the price of benzene25 despite containing two fewer carbon atoms. Among the catalyst deactivation mechanisms mentioned above, alkali metal poisoning is facile to mimic and also has the best potential as a route to make catalysts tunable to a desired product distribution. The role of biomass derived alkali metals, particularly potassium, on influencing primary biomass pyrolysis reactions has been well studied. Its role in cellulose decomposition is to favor the production of small oxygenates such as hydroxyacetaldehyde, acetol, and formic acid over levoglucosan and furans.26,27 Its effect on CFP over fresh HZSM-5 has been to decrease the yield of aromatic hydrocarbons, which is mostly ascribable to effects on the initial pyrolysis processes. 28−30 In a study of catalyst deactivation, our group found that in the CFP of switchgrass over HZSM-5, the alkyl phenol content of the bio-oil increased with metal accumulation (with a high proportion of K) on the catalyst to a level well above what is found for noncatalytic pyrolysis.13 These increases were concurrent with a decrease in aromatic hydrocarbon yield, but not with large increases in other primary pyrolysis products such as levoglucosan or methoxy phenols. This suggests that production of alkyl phenols may be selected for by modification of the zeolite with alkali metals such as potassium. A recent report has found that one source of alkyl phenols may be the release of intermediates to aromatics from the catalyst active site via reaction with water.31 This mechanism provides a path for production of alkyl phenols from not only the lignin portion of the biomass but also the cellulose, which has been observed by our group as well.29 The potential convergence of the product suite from both holocellulose and lignin increases the viability of these compounds as potential coproducts. In this study, two ZSM-5 materials at two different SiO2/ Al2O3 ratios were exchanged with potassium and compared to the parent HZSM-5 zeolite. We focused on the production of alkyl phenols and furans as potential oxygenated chemical products that could be produced during catalytic pyrolysis to complement the production of aromatic hydrocarbons. The effects of these variations in the catalysts were studied for cellulose, hemicellulose (xylan), lignin, and switchgrass in an effort to understand the role of the catalyst-bound K on the conversion of each with the goal of generating data that may be useful to designing future catalysts to better select for these catalytically produced functionalized products.

Research Article

METHODS

Materials. Cellulose, xylan, KNO3, silica (SiO2), and authentic compounds employed as standards for GC-MS calibration were purchased from Sigma-Aldrich and used as received. NH4ZSM-5 (CBV-2314; 23 SiO2:Al2O3 and CBV-5514 55 SiO2:Al2O3) was purchased from Zeolyst International (Conshohocken, PA). Lignin was purchased from Granit Research and Development SA. Switchgrass was provided by the McDonnel Farm (East Greenville, PA) and finely ground and dried prior to use. Catalyst Preparation. The potassium substituted catalysts were prepared via ion exchange by vigorously stirring a suspension of the parent zeolite in an aqueous solution of KNO3 under reflux conditions overnight. The catalyst was then filtered, washed with DI H2O, dried, and calcined at 550 °C. K content was measured by ICP and performed by Galbraith Laboratories, Inc. (Knoxville, TN). K contents of the catalysts used and Bronsted acidities are reported in Table 1.

Table 1. Potassium Content and Acidity of Catalysts Used in this Study catalyst

SiO2/ Al2O3

HZSM-5 (23) KZSM-5 (23) HZSM-5 (55) KZSM-5 (55)

23 23 55 55

K (wt %) 2.95 1.35

approx. Brønsted acidity (μmol/g)a 1022 250 500 155

a

Brønsted acidity measured by isopropylamine thermal programmed desorption for HZSM-5 (23)15 and estimated for other catalysts by change in the Si/Al ratio and K exchange efficiency.

Although not measured, it is expected that the Lewis to Bronsted ratio will increase as potassium content in the sample increases.32 K/SiO2 used for control experiments (data in Supporting Information) was produced by stirring a solution of KNO3 with suspended SiO2 in proportion such that the K content would be 3 wt %. The solution was then distilled to dryness via rotary evaporation. Pyrolysis Experiments. Micropyrolysis was performed on a Frontier Lab Double-Shot micro pyrolyzer PY-3030iD with a Frontier Lab Auto-Shot Sampler AS-1020E attached to a gas chromatograph, Shimadzu GC-2010. Detection of products was achieved using a Shimadzu GCMS-QP2010S mass spectrometer. The interface temperature of the micropyrolyzer was set to 300 °C, and the furnace was set to 500 °C. A sample size of ∼0.3 mg of biomass and catalyst (amount varies with ratio studied) was subjected to a single-shot pyrolysis at the desired temperature for 30 s using stainless steel cups (disposable ecocup LF; Frontier Laboratories) followed by separation on the GC. Biomass was loaded into the cup first, and catalyst was added on top. Analysis of condensable gas was performed on an RTX-1701 60 m × 0.25 mm GC fused silica capillary column with a 0.25 μm film thickness. The oven for the GC column was set at an initial temperature of 45 °C for 5 min followed by a ramp rate of 3 °C min−1 to 280 °C and held for 20 min for a total run time of 102 min. The injector temperature was at 250 °C with a split ratio of 90:1 and a helium flow rate of 1 mL min−1. For the analysis of noncondensable gases, identical experiments were performed with a different column and GC method. A split ratio of 100 and a CP-PoraBOND Q 25 m × 0.25 mm fused silica capillary column was used (Varian, Palo Alto, CA). The oven for the GC column was set at 35 °C for 3 min followed by a ramp rate of 5 °C min−1 up to 150 °C then 10 °C min−1 to 250 °C and held for 10 min. Quantitative analysis of the yield of individual chemical products was done according to the external standard method, using authentic samples to generate calibration curves.

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RESULTS AND DISCUSSION During CFP of biomass, it is well-known that most acidic catalysts, those with the lowest SiO2/Al2O3 ratios, produce the highest yield of aromatic hydrocarbons.11 However, they also lead to higher coke yields and more rapid deactivation.14 For 2155


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Figure 1. Comparison of yields from selected compounds from switchgrass (SWG) and cellulose (Cell) over HZSM-5 (23) and KZSM-5 (23) at 5:1 catalyst/biomass and 500 °C. Alkyl phenols = phenol, o-,m-,p-cresols, 2,4-dimethyl phenol, and 4-ethyl phenol. Methoxy phenols = guaiacol, 4methylguaiacol, syringol. BTEX = benzene, toluene, ethylbenzene, xylenes. Error bars represent one standard deviation.

Figure 2. Comparison of yield of phenolics, furans, and BTEX from CFP over KZSM-5 (23) of switchgrass with catalyst loading (500 °C). See the Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

batch reactors, such as the micropyrolyzer used here, temperatures of about 500 °C and high catalyst to biomass ratios (from 5:1 to 15:1) are used to achieve full conversion to aromatic hydrocarbons, with BTEX compounds (benzene,

toluene, ethylbenzne, and xylenes) usually being the most desired.5,11 Therefore, the effect of potassium exchange was first studied under these conditions (catalyst/biomass = 5, 500 °C). As shown in Figure 1, when the KZSM-5 (23) catalyst was used 2156


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Figure 3. Varation in yield of selected CFP over KZSM-5 (23) products from switchgrass with temperature (2.5:1 catalyst/biomass ratio). See Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

for the CFP of switchgrass, there was a 74% decrease in the production of BTEX compounds and ∼3-fold increase in the production of the selected alkyl and methoxy phenols compared to the parent HZSM-5. In the case of methoxy phenols, the yield was lower than that achieved noncatalytically, which suggests they were not formed catalytically, but rather their catalytic conversion to other products was limited by the use of a less active catalyst. However, the alkyl phenol yield was substantially higher (>4-fold increase) than that produced noncatalytically, meaning the alkyl phenols were the product of a catalytic reaction, not merely a consequence of diminished catalytic conversion. Similarly, there was an increase in the production of furans, particularly 2-methylfuran (furan was also detected but not quantified), whose production increased to a yield of 4.6 mg/g, whereas only trace amounts were observed for HZSM-5 (23) and noncatalytic pyrolysis (∼0.4 mg/g each). Furfural (furan-2-carboxaldehyde) also increased in production, although its yield was less than what was produced noncatalytically. Furfural may be a less desired compound because it is an aldehyde and is known to contribute to bio-oil thermal instability.33 To consider the change in the conversion of primary pyrolysis oxygenates, acetol (2-hydroxyacetone) was used as a representative compound. Both catalysts provided significant conversion of 4.3 mg/g of acetol produced noncatalytically; although, the KZSM-5 (23) did provide a lesser level of conversion (1.1 mg/g acetol yield) than did HZSM-5 (23) (0.3 mg/g acetol yield). When cellulose was used as the starting material rather than switchgrass, the same trend is observed, and the production of alkyl phenols from cellulose is verified. Alkyl phenols were produced in a yield of 8.9 mg/g from cellulose over KZSM-5 (23), compared with 3.8

mg/g over HZSM-5 (23) and only trace amounts (∼0.1 mg/g) noncatalytically. Similarly, the yield of 2-methyl furan increased to over 5 mg/g over KZSM-5 (23), compared with ∼1 mg/g using HZSM-5 (23). Also, the conversion of acetol followed the same trend with yields decreasing from 1.4 mg/g noncatalytically to 0.3 mg/g and 0.6 mg/g over HZSM-5 (23) and KZSM5 (23), respectively. Hypothesizing that the high catalyst/biomass ratio conditions developed for the production of aromatic hydrocarbons may not be optimum for production of these catalytically produced oxygenates, the use of higher and lower catalyst to biomass ratios was explored (Figure 2) for the CFP of switchgrass. For the production of alkyl phenols, lowering the catalyst/biomass ratio proved to be more effective than increasing it and also led to significantly decreased BTEX production. As was expected, the BTEX yield did increase at higher loading of the KZSM-5 (23). Total phenolic yield was optimized at a catalyst/biomass ratio of 2.5:1, with the alkyl phenols yield similar at both the 2.5:1 and 5:1 ratios but methoxy phenol production greater at 2.5:1. Production of 2methylfuran was also optimized at this catalyst/biomass ratio (4.2 mg/g), a 10-fold increase over the noncatalytic yield. Alkyl phenol and 2-methylfuran production drop off dramatically when the catalyst/biomass ratio is lowered to 1:1. However, the furfural yield is maximized under this condition (11.1 mg/g). At a catalyst loading of 2.5:1, the total yield of the selected phenolic compounds reached 8.75 mg/g of switchgrass input, which is a 125% increase over the noncatalytic case (3.9 mg/g). The alkyl phenol yield of 5.8 mg/g is nearly 5-fold that of the noncatalytic experiments (1.2 mg/g). 2157


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Figure 4. Production of selected CFP products from cellulose over HZSM-5 and KZSM-5 catalysts (2.5/1 catalyst/biomass ratio, 500 °C). See Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

Figure 5. Production of selected CFP products from xylan over HZSM-5 and KZSM-5 catalysts (2.5/1 catalyst/biomass ratio, 500 °C). See Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

Al2O3 = 55 (lower acidity) was prepared. The KZSM-5 (55) had a lower total K content (Table 1) because it had fewer H+ sites available for exchange. Each of the four catalysts were tested for their effects on the production of BTEX, phenols, 2methylfuran, and noncondensable gases using the optimum conditions from the above discussion (2.5:1 catalyst/biomass ratio, 500 °C). Each of the three biopolymers making up biomass cellulose, hemicellulose (xylan used as surrogate), and lignin along with switchgrass were tested to determine the effect of K+ and the Brønsted acidity of the catalyst on each component. For cellulose (Figure 4), similar production of BTEX was found for the two HZSM-5 catalysts, but the production was greatly decreased in the case of the zeolites exchanged with K. The desired alkyl phenols (methoxy phenols are not produced from carbohydrate portions of the biomass) were actually maximized at 7.1 mg/g with the HZSM-5 (55), suggesting that

Again noting that the usual conditions for optimizing production of aromatic hydrocarbons may be different to optimize the yield of these oxygenates, the effect of temperature was studied for the 2.5:1 catalyst/biomass ratio (Figure 3). For switchgrass, using a lower temperature (400 °C) decreased the yield of both BTEX and phenolic compounds. The yields remained similar upon increasing the temperature to 600 °C. However, for cellulose the yield of alkyl phenols increased as the pyrolysis temperature increased, with the yield of alkyl phenols reaching a maximum at 600 °C (7.9 mg/g, data not shown). This may suggest that there are competing pathways for the production of alkyl phenols, with the path from cellulose enhanced at higher temperatures and the path from lignin (via demethoxylation) suppressed at higher temperatures. To better understand the role of potassium exchange on the catalytic function and activity toward production of desired oxygenates, a second series of zeolite catalysts at a higher SiO2/ 2158


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Figure 6. Production of selected CFP products from lignin over HZSM-5 and KZSM-5 catalysts (2.5:1 catalyst/biomass ratio 500 °C). See Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

Figure 7. Production of selected CFP products from switchgrass over HZSM-5 and KZSM-5 catalysts (2.5:1 catalyst/biomass ratio, 500 °C). See Figure 1 caption for compound group definitions. Error bars represent one standard deviation.

attenuation of the zeolite Brønsted acidity may be a more important factor than the presence of the potassium. Each of the K-substituted zeolites produced higher amounts of alkyl phenols than the highly acidic HZSM-5 (23), although the production dropped off as the acidity decreased from the 500 μmol/g of the HZSM-5 (55). The yield of 2-methyl furan was highest with the two K-exchanged catalysts, and the highest yield was achieved with KZSM-5 (55), the least acidic catalyst. The trends are similar for xylan, although overall alkyl phenols and 2-methyl furan are produced in lower yields (Figure 5). Also, the production of alkyl phenols is highest with KZSM-5 (55) with an acidity of 250 μmol/g. In this case, similar yields were produced using HZSM-5 (55) and KZSM-5 (55) with acidities of 500 and 150 μmol/g, respectively. This suggests that moderate acidity at low catalyst loading is the best condition for the production of alkyl phenols. The role of K may only be to reduce the number of Bronsted acid sites. However, in the case of 2-methyl furan, lower acidity and the presence of K appears

to be important. In control experiments of catalytic pyrolysis of cellulose over SiO2 (Table S1), alkyl phenols and 2-methyl furan were produced in higher yield over SiO 2 than noncatalytically but at much lower levels than any of the ZSM-5 catalysts. The addition of K to the SiO2 slightly decreased the alkyl phenol and 2-methyl furan yield, showing the importance of the ZSM-5 zeolite structure and acidity to the catalytic activity. When lignin is the feedstock, the two moderately acidic catalysts HZSM-5 (55) and KZSM-5 (23) maximize the yield of alkyl phenols, while the two K-exchanged zeolites retain the highest yields of methoxy phenols. Overall, KZSM-5 (23) produces the highest yield of total phenols (Figure 6). For switchgrass, the results are reflective of the individual components, with the yield of alkyl phenols highest with the two moderately acidic catalysts, while methoxy phenols and 2methyl furan yields were highest with the K-exchanged catalysts 2159


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Table 2. Yields (wt%) of Noncondensable Gases (NCGs) from the CFP of Biomass Components and Switchgrass via CFP over HZSM-5 and KZSM-5 Catalysts (2.5/1 Catalyst/Biomass, 500 °C) CO

CH4

CO2

C2H4

cellulose xylan lignin switchgrass

1.47 3.30 1.06 1.79

0.02 0.10 1.06 0.17

2.92 17.34 7.36 6.39

0.05 0.04 0.09 0.06


8.10 6.74 1.83 7.76

0.01 0.05 0.84 0.17

5.63 14.48 6.36 8.09

0.24 0.57 0.76 0.58


10.35 7.05 2.18 8.14

0.01 0.05 0.90 0.17

7.23 16.09 6.76 8.45

0.19 0.40 0.73 0.41


6.47 5.06 1.59 4.77

0.01 0.09 0.88 0.17

7.13 16.48 6.97 8.50

0.07 0.14 0.20 0.15


7.71 4.87 1.34 4.69

0.03 0.07 0.79 0.20

7.97 16.54 5.26 7.79

0.08 0.16 0.16 0.13

C2H6 non-catalytic 0.01 0.07 0.09 0.02 HZSM-5 (23) 0.00 0.05 0.08 0.00 HZSM-5 (55) 0.00 0.08 0.08 0.01 KZSM-5 (23) 0.00 0.07 0.07 0.01 KZSM-5 (55) 0.00 0.07 0.07 0.00

C3H6

C3H8

total NCGs

0.03 0.06 0.08 0.05

0.00 0.45 0.36 0.03

4.49 21.35 10.10 8.51

0.16 0.50 0.79 0.57

0.01 0.13 0.30 0.05

14.17 22.52 10.94 17.22

0.20 0.43 0.87 0.54

0.00 0.07 0.15 0.03

17.98 24.16 11.67 17.74

0.23 0.40 0.69 0.40

0.00 0.25 0.03 0.01

13.92 22.49 10.42 14.01

0.22 0.33 0.51 0.32

0.00 0.31 0.03 0.00

16.01 22.36 8.16 13.14

Figure 8. Production of alkyl phenols from primary pyrolysis oxygenates via K/HZSM-5 zeolite catalysts.

formation of alkyl phenols trends away from the formation of 2methyl furan, likely because one pathway to their formation is consumption of furans by a Diels−Alder reaction with olefins produced during pyrolysis.34 Once the Diels−Alder intermediate (A) is formed, it can be dehydrated to monoaromatic hydrocarbons or undergo dehydrogenation to phenols. The dehydration would be a Brønsted acid catalyzed reaction, which is consistent with the observed trend of increasing alkyl phenols with decreasing zeolite acidity. Another reported mechanism for alkyl phenol formation during catalytic pyrolysis involves the oxidation of aromatic hydrocarbon intermediates by water,31 which would be lessened in the case of the Kexchanged catalysts yet still active in the case of the HZSM-5

(Figure 7). BTEX yields are greatly decreased in the presence of the K-substituted zeolites. To gain more insight into the role of the catalyst properties on the chemical pathways to the products, it is instructive to look at the production of the noncondensable gas coproducts (Table 2). In particular, the trends in CO yields are important, as decarbonylation is a major pathway in ZSM-5 CFP. For all of the feedstocks studied, the CO production is inhibited by the addition of potassium. As shown in Figure 8, this is consistent with a decrease in BTEX formation as decarbonylation of oxygenates is the source of olefins that make up the “hydrocarbon pool” that is the precursor to aromatic hydrocarbons.6 Furthermore, it is consistent with the increase in 2-methyl furan as it is less likely to enter this pathway. The 2160


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(4) Liu, C.; Wang, H.; Karim, A. M.; Sun, J.; Wang, Y. Catalytic fast pyrolysis of lignocellulosic biomass. Chem. Soc. Rev. 2014, 43, 7594− 7623. (5) Carlson, T. R.; Vispute, T. P.; Huber, G. W. Green Gasoline by Catalytic Fast Pyrolysis of Solid Biomass Derived Compounds. ChemSusChem 2008, 1, 397−400. (6) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W.Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222−1227. (7) Jae, J.; Coolman, R.; Mountziaris, T. J.; Huber, G. W. Catalytic fast pyrolysis of lignocellulosic biomass in a process development unit with continual catalyst addition and removal. Chem. Eng. Sci. 2014, 108, 33−46. (8) Zhang, H.; Xiao, R.; Jin, B.; Shen, D.; Chen, R.; Xiao, G. Catalytic Fast Pyrolysis of straw biomass in an internally interconnected fludized bed to produce aromatics and olefins: effect of different catalysts. Bioresour. Technol. 2013, 137, 82−87. (9) Williams, P. T.; Nugranad, N. Comparison of Products from the Pyrolysis and Catalytic Pyrolysis of Rice Husks. Energy 2000, 25, 493− 513. (10) Zhang, H.; Xiao, R.; Huang, H.; Xiao, G. Comparison of NonCatalytic and Catalytic Fast Pyrolysis of Corncob in a Fluidized Bed Reactor. Bioresour. Technol. 2009, 100, 1428−1434. (11) Mihalcik, D. L.; Mullen, C. A.; Boateng, A. A. Screening Acidic Zeolites for Catalytic Fast Pyrolysis of Biomass and its Components. J. Anal. Appl. Pyrolysis 2011, 92, 224−232. (12) Mullen, C. A.; Boateng, A. A.; Mihalcik, D. L.; Goldberg, N. M. Catalytic Fast Pyrolysis of White Oak Wood in a Bubbling Fluidized Bed. Energy Fuels 2011, 25, 5444−5451. (13) Mullen, C. A.; Boateng, A. A. Accumulation of Inorganic Impurities on HZSM-5 Zeolites during Catalytic Fast Pyrolysis of Switchgrass. Ind. Eng. Chem. Res. 2013, 52, 17156−17161. (14) Wan, S.; Waters, C.; Stevens, A.; Gumidyala, A.; Jentoft, R.; Lobban, L.; Resasco, D.; Mallinson, R.; Crossley, S. Decoupling HZSM-5 Catalyst Activity from Deactivation during Upgrading of Pyrolysis Vapors. ChemSusChem 2015, 8, 552−559. (15) Mullen, C. A.; Boateng, A. A. Production of Aromatic Hydrocarbons via Catalytic Pyrolysis of Biomass over Fe-Modified HZSM-5 Zeolites. ACS Sustainable Chem. Eng. 2015, 3, 1623−1631. (16) Cheng, Y.; Jae, J.; Fan, W.; Huber, G. W.; Shi, J. Production of Renewable Aromatic Compounds by Catalytic Fast Pyrolysis of Lignocellulosic Biomass with Bifunctional Ga/HZSM-5 Catalysts. Angew. Chem., Int. Ed. 2012, 51, 1387−1390. (17) Iliopoulou, E. F.; Stefanidis, S. D.; Kalogiannis, K. G.; Delimitis, A.; Lappas, A. A.; Triantafyllidis, K. S. Catalytic upgrading of biomass pyrolysis vapors using transition metal modified ZSM-5 zeolite. Appl. Catal., B 2012, 127, 281−290. (18) Veses, A.; Puértolas, B.; Callén, M. S.; García, T. Catalytic upgrading of biomass derived pyrolysis vapor over metal-loaded ZSM5 zeolites: Effect of different metal cations on the bio-oil final properties. Microporous Mesoporous Mater. 2015, 209, 189. (19) Schultz, E. L.; Mullen, C. A.; Boateng, A. A. Aromatic hydrocarbon production from Eucalyptus urophylla pyrolysis over several metal-modifed ZSM-5 catalysts. Energy Technol. 2017, 5, 196. (20) Cheng, Y.; Wang, Z.; Gilbert, G. J.; Fan, W.; Huber, G. W. Production of p-Xylene from Biomass by Catalytic Fast Pyrolysis Using ZSM-5 Catalysts with Reduced Pore Openings. Angew. Chem., Int. Ed. 2012, 51, 11097−11100. (21) Yildiz, G.; Ronsse, F.; Venderbosch, R.; van Duren, R.; Kersten, S. R.A.; Prins, W. Effect of biomass ash in catalytic fast pyrolysis of pine wood. Appl. Catal., B 2015, 168−169, 203−2011. (22) S&P Global Platts. http://www.platts.com/news-feature/2015/ petrochemicals/global-solvents-overview/solvents-phenol-prices. (23) Elkasabi, Y.; Mullen, C. A.; Boateng, A. A. Aqueous Extractive Upgrading of Bio-Oils Created by Tail-Gas Reactive Pyrolysis To Produce Pure Hydrocarbons and Phenols. ACS Sustainable Chem. Eng. 2015, 3, 2809−2816.

(55), perhaps explaining why alkyl phenol yields were slightly higher for this catalyst, particularly from cellulose.

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CONCLUSION In this study, modification of ZSM-5 type zeolties was performed via ion exchange with potassium to prepare a partially “deactivated” catalyst to produce valuable oxygenated compounds, particularly alkyl phenols and 2-methylfuran, as potential coproducts to the formation of aromatic hydrocarbons. Yields of both of these classes of compounds were found to be increased ∼3−4-fold over the yields for noncatalytic pyrolysis and catalytic pyrolysis using highly acidic HZSM-5 catalysts. Low catalyst loadings and lower acidity were found to favor production of alkyl phenols, whereas low catalyst loading along with K incorporation were found to increase the yield of 2-methyl furan. Although the optimized absolute yields of the compounds obtained were modest, the observations will be useful in designing catalysts and conditions to increase yields toward levels that are economically relevant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02262. Results from control experiments using SiO2 and K/SiO2 as catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-215-836-6916. E-mail: Charles.Mullen@ars. usda.gov. ORCID

Charles A. Mullen: 0000-0001-5739-5451 Notes

Disclaimer. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Frankie Lazauskas, co-op student from Drexel University, for technical assistance. Funding from USDA-NIFA-BRDI Grant No. 2012-10008-20271 is hereby acknowledged.

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REFERENCES

(1) U.S. Department of Energy. 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Vol. 1: Economic Availability of Feedstocks; Langholtz, M. H., Stokes, B. J., Eaton, L. M. (Leads); ORNL/TM-2016/160; Oak Ridge National Laboratory: Oak Ridge, TN, 2016. (2) Talmadge, M. S.; Baldwin, R. M.; Biddy, M. J.; McCormick, R. L.; Beckham, G. T.; Ferguson, G. A.; Czernik, S.; Magrini-Bair, K. A.; Foust, T. D.; Metelski, P. D.; Hetrick, C.; Nimlos, M. R. A perspective on oxygenated species in the refinery integration of pyrolysis oil. Green Chem. 2014, 16, 407−453. (3) Bridgewater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012, 38, 68−94. 2161


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Role of Potassium Exchange in Catalytic Pyrolysis of Biomass over

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